There are different duplicating processes which depend on positioning an image on a printing substrate such as a copy sheet in accurate alignment with a predetermined reference on a plate or master cylinder and adjusting the image position relative to the reference, if necessary, in order to achieve requisite printing quality on the substrate. One such duplicating process is the offset duplicating process. Commonly, a master carrying assembly, for example, a cylinder, serves to hold the master or plate during the cyclical operation that effects the transfer of images to the copy paper. The head end of the master is held securely by the master carrying assembly. The holding mechanism may take various forms including one form which clamps a master end. Another form of the master holding or securing mechanism may have a series of pins projecting from a bar extending generally parallel to the cylinder axis and mounted adjacent the cylinder periphery. The master end holding means, whatever its form may be, is preferably adjustable, both angularly and perpendicularly, relative to the direction of movement of the master carrying assembly to suitably align the print on the copy sheet. This is because the orientation of the image on the master, for example the lines of print across the width of the master, are not necessarily precisely related to the master end or to the perforations in the master, which are the portions of the master engaged by the master holding means. To explain further, a clamp commonly has a straight edge against which one end of the master is pressed before the master is gripped to align the master relative to the carrying assembly. If the end of the master is not parallel to the lines of print, then the duplicated image will be skewed on the copy sheet. In the other holding means referred to, a pin bar is provided to engage the perforations. It is understandable how an image may be misaligned on a copy sheet if the perforations or pin holes along the edge of a master are not aligned properly relative to the image on the master.
Prior art devices have structures which adjust the alignment of the pin bars. One example of such structure is a thumb screw positioned to one side of a center pivot for the beam which connects the pin bar to the frame of the master carrying assembly. The thumb screw has posts on either side extending in opposite directions which have left-hand and right-hand threads, respectively. One post is threaded in a fixed support and the other is threaded in a support which is a part of the pivotable beam. Turning of the screw draws one side of the beam toward the fixed support or forces that side of the beam away from the fixed support, depending on which direction the thumb screw is turned, thereby pivoting the beam and the pin bar with it. It is in this way that the master is aligned. Such a structure is adapted for installation on the A. B. Dick Company (assignee of the present invention) Model 360 offset press.
Commonly assigned U.S. Pat. No. 4,459,913 provided needed corrections to the prior art and discloses a master carrier having a surface overlayed by a master sheet and which carrier has an assembly for clamping the straightedge of the master, such clamping assembly being provided with an improved mechanism for adjusting the position of the master sheet relative to overlayed carrier surface without requiring the master sheet to be removed. Thus in that patent, the invention provided an improved master clamping mechanism which was relatively easy to operate, which permitted angular adjustment of the universal master clamp and which was reliable.
There are other problems concerned with duplicating mechanisms. First, when the leading edge clamping assembly is adjusted from both sides, it requires a high degree of operator skill to properly position the leading edge clamp assembly to correct the problem of skewing. In addition, once the leading edge clamp assembly has been adjusted, then the trailing edge clamp assembly must be separately adjusted to compensate for the movement of the tail end of the plate or master which has moved because of the adjustment of the leading edge clamp assembly. Also, the prior art does not make any effort to correct for varying plate thicknesses except by using the operator with a high degree of skill to adjust the proper pressure of the clamp assembly to accommodate varying sizes or thicknesses of the plates or masters.
The first problem, adjusting of both the leading and trailing edge plate clamps, has been considered in the prior art. German patents Nos. 893,343, 0,401,500, and 1,536,954 and U.S. Pat. No. 4,785,736 all disclose mechanisms for adjusting the leading edge and trailing edge plate clamps. In German patent No. 893,343, it appears that each of the leading edge and trailing edge plate clamps can be both rotated about a center pivot point and axially moved in either direction. They can also cause both the leading edge and trailing edge plate clamp assemblies to move axially in the same direction. German patent No. 0,401,500 discloses a mechanism for adjusting the leading and trailing edge clamps by rotating two separate pivot levers, one at each end of the plate clamps, that are coupled to both plate clamps. When the levers are rotated, one plate clamp moves axially in one direction and the other plate clamp moves axially in the other direction simultaneously. German patent No. 1,536,954 has a similar mechanism except that a single lever is coupled to the center of each plate clamp and is rotatably mounted to the frame at the center point of the lever. Adjusting the lever to move the leading edge plate clamp axially in one direction simultaneously moves the trailing edge plate clamp in the opposite direction. The device in U.S. Pat. No. 4,785,736 operates in a manner similar to the operation of German patent No. 0,401,500. In each of these patents, both the leading edge and trailing edge plate clamps are required to move axially, or axially and angularly, thus increasing the difficulty of the adjustment and the complexity of the adjusting mechanism.
The present invention overcomes the disadvantages of the prior art by providing a leading edge plate holding or clamp assembly which is formed with either a cantilevered or pivoted beam which has first and second register pins thereon and a calibrated adjustment fixture that adjusts the leading edge plate causing one of the register pins to move with respect to the other and enabling skewing of the leading edge of the master or plate.
In addition, a linkage exists between the leading edge clamp assembly and the trailing edge clap assembly such that adjusting the leading edge plate clamp assembly to skew the leading edge of the plate automatically adjusts the trailing edge clamp assembly in proper alignment with the trailing edge of the skewed master or plate.
Finally, spring means are interposed between the plate cylinder and each of the clamps for automatically compensating for variation in the thickness of the plate being clamped simply by the operator moving a control lever from a first position to a second position. Thus, no individual operator adjustment is required. The operator simply opens the clamp or closes it and it automatically adjusts to compensate for plates or masters of variable thickness.
Thus, it is one aspect of the present invention to provide a clamping or holding assembly for a duplicating machine such as a printing press or the like which utilizes a cantilever or pivoted beam that is adjusted to enable skewing of the plate as desired.
It is another aspect of the present invention to provide a coordination of the leading and trailing edge plate clamp assemblies such that adjustment of the leading edge plate clamp assembly for skew automatically adjusts the trailing edge plate clamp assembly accordingly.
It is still another aspect of the present invention to provide a plate clamp assembly for a duplicating machine such as a printing press or the like which is self-adjusting for different plate thicknesses.
It is yet another aspect of the present invention to provide a tensioning means for the trailing edge of the plate such that the plate is held uniformly tight against the cylinder in its skewed orientation.